Black holes: Formation & Properties

Black holes are regions of space-time that prevent anything, including light, from escaping. The reason for this is that black holes are extremely dense and so the escape velocity required to escape the black hole must be huge. It should be made clear that a black hole is a distortion of space-time (Einstein had described space as a fabric that can be distorted and results in gravity). Mathematics involving the region around a black hole have shown us that there is a defined surface (the event horizon) that acts as an information barrier. This means that once an object passes this event horizon, there is no coming back. A crucial point to be made is that a black hole is formed at the end of a star’s life.

A key person who has contributed to our understanding of black holes has been Karl Schwarzschild. In 1916, Schwarzschild discovered the first modern solution of general relativity that would characterize black holes. The way we observe black holes is by observing how electromagnetic radiation, light, interacts at black holes. It is of great importance and also difficulty to determine how black holes exactly form and evolve over time. One thing I find very interesting about astronomy is how physicists are able to determine physical phenomena that is so far away and able to come up with mechanisms for that particular phenomena.

How exactly does a black hole form? What is it made out of? What properties make a black hole distinct? These are all fundamental questions that scientists, including Stephen Hawking, have spent countless decades working on in order to better understand the universe. We know that a black hole occurs at the end of a star’s life. But what exactly is the mechanism that a relatively huge star condenses into a black hole of extraordinary gravity?

The mechanism through which a star becomes a black hole is called gravitational collapse. We know that every object in the universe experiences some magnitude of gravity. What keeps the object from collapsing upon itself? In the case of stars, it’s the internal pressure that matches the gravity acting on that object to prevent collapse. However, when that internal pressure is no longer sufficient to match the force of gravity, the star collapses on itself. This usually occurs when the star is running low on fuel, which when burned helps to create pressure. When this gravitational collapse occurs, a lot of energy is released in the process. However, if there was an outside observer who was to observe this phenomenon, they would not witness this process. The reason for this is that the observer would see the falling matter slow and stop just before the event horizon (gravitational time dilation).

Once a black hole has formed, it is able to grow through the addition of mass. It is possible for 2 black holes to merge together to create supermassive black holes. An important discovery made by Stephen Hawking in 1974 has to do with the emission of small bits of thermal radiation from black holes. Previously, it was thought that black holes are entirely black, however, Hawking showed, through quantum field theory, that a black hole does emit particles in a perfect black body spectrum. Larger black holes emit less radiation than compared to smaller black holes, which emit more. This emission is responsible for the evaporation of black holes. Therefore, the more radiation (smaller black holes) emitted, the faster the rate of evaporation.

It is very hard to detect black holes from Earth just because there is no known mechanism to detect them. A way to maybe measure black holes is to look out for the instance where a flash of light is given off due to Hawking radiation. However, the problem with this is that the time scale is so short, it is hard to measure.